Co-founded by Billie Jean King, the co-ed U.S. tennis circuit kicks off its 32nd season on Thursday.
The month-long series will feature appearances by Venus and Serena Williams, Maria Sharapova and the top-ranked doubles duo of Bob and Mike Bryan.
King and WTT commissioner Ilana Kloss have been innovators in the sport -- debuting instant replay, on-court coaching and no-ad scoring -- and will launch a "Go Green" effort in two of the league's 11 American cities.
"We're trying to take a leadership role in going green," Kloss said of the energy-conscious endeavor with FirmGreen.
"We know it's a huge job. We're committed to start in California with the two teams in Sacramento and Newport Beach."
Kournikova will headline the Sacramento Capitals' home opener on Thursday against the Springfield (Missouri) Lasers.
It's her fifth season in WTT and third straight with the Capitals.
"It's awesome that tennis gets to be displayed in cities where normally you don't have big tournaments," Kournikova said.
"It's so friendly for the fans. You get to see the current players. You get to see the young and up and coming. So it's just a diversity."
The youngest is 14-year-old Michelle Larcher de Brito, who upset Meghann Shaughnessy earlier this year.
McEnroe, 48, is the oldest of the 53 players from 18 countries in the league.
